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Crossed polarized light photomicrograph of silliminite whowing one principle textural type: needle-like fibrolite from a quartz-muscovite-silliminite-biotite schist. Mag. 10x. |
| Formula | Al2SiO5 | Most sillimanite is relatively pure, although minor amounts of Fe3+, Cr 3+, Ti 4+ may be present. |
| Crystal System | orthorhombic | a=7.488, b=7.684, c=5.777. |
| Crystal Habit | Slender prismatic or fine fibrous crystals. | Masses of fine fibrous crystals, known as fibrolites, may form swirled or matted aggregates. |
| Cleavage | Perfect on (010) |
Uneven fracture. Brittle. Fibrous aggregates may be tough. |
| Color/Pleochroism | Colorless or white, less commonly yellow, brown or blue. | Vireous luster and white streak. |
| Optic Sign | Biaxial (+) | |
| 2V | 20-30 |
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| Optic Orientation | X=a Y=b Z=c |
Optic axis dispersion trong, r > v. Fragments on (010) cleavage yield flash figures. |
| Refractive Indices alpha = beta = gamma = delta = |
1.653-1.661 1.657-1.662 1.672-1.683 0.018-0.022 |
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| Max Birefringence | 0.018-0.022 | Indices of refraction show little variation. Interference colors in thin section, are up to lower second order. Fine fibrolite fibers show lower inerference colors because they are hinner than a standard thin section. |
| Elongation | length slow | |
| Extinction | parallel | |
| Distinguishing Features | High relief, moderate birefringence, parallel extiction, and slender prismatic or fibrous crystal habit. | |
| Occurrence | Sillimanite occurs as a constituent of high-temperature metamorphosed rocks containing clay. In contact metamorphosed rocks it may occur in silimanite-cordierite gneisses or sillimanite-biotite horfels. In silica poor rocks, it may be associated with corundum. | |
| Editors | Mai Kobayashi ('06) | |
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Crossed polarized light photomicrograph of silliminite whowing one principle textural type: needle-like fibrolite from a quartz-muscovite-silliminite-biotite schist. Mag. 10x. |